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Minimum Voltage at incoming supply point

Retired now and not up to date with reg’s since 16th. Helping advise a friend. Currently they have 100amp single phase supply and whilst the no load voltage is up at around 238 volts, as the circuit is loaded, voltage drops off substantially. Even with a load of 50 amps, ie, half the supply fuse rating, voltage is slightly below the -6% limit, at around 215 volts, further load simply pushes voltage significantly lower, and by extrapolating measured data, it would be nearer 170 volts with a load of 100 amps, is this acceptable? I plan measuring Ze and pfc this week as a guide perhaps to supply loop impedance and, which I suspect must be causing this excessive drop. I’m aware of the ESQCR reg’s although need to update myself, but are there any other regulations that apply?

  • It sounds unacceptably low.

    Before complaining though, do try and ascertain what the agreed supply capacity is. It might be a lot less than is implied by the 100 amp cut out fuse holder.

    Have you seen the fuse rating ? It might be a lot less than 100 amps. Or the fuse fitted might be larger then it should be.


    If the supply voltage is persistently below the minimum with the load not exceeding that agreed, then there would seem to be cause for complaint to the DNO.

    I presume that the voltage was measured at the point of supply, or reasonably close thereto and not at the far end of a long or marginally sized customer owned sub main. And that the instrument used is preferably calibrated recently.
  • Thanks Broadgage. And yes, I had been wondering about the supply being limited to below the 100 amps  (but even if only 60 amps, voltage is still out of spec) of the intake fuse currently fitted, and I will be seeking to clarify this.  (it’s a very long run to the supply transformer, with old lead sheathed cables too, so likely to be TN-S). Yes, voltage was measured at supply input. DNO engineer has very recently collected a weeks worth of site voltage and current data, and in my opinion, has misinterpreted the data , and I will be strongly questioning this. First though, I am meeting a particularly reliable electrician on site today (one of few that I could rely on during my time as a Council Electrical Contracts engineer/manager, one of my "hobbies" after retiring early from main engineering career) to use his recently calibrated tester, to measure Ze and the pfc, and I’ll get him to also compare measured voltage with that recorded by my multimeter used (although not recently calibrated, I do regularly compare it with some of my other multimeters, to get confidence in the indications). 

  • Welcome.


    That is very droopy!  20V -25V drop for 50A load is  about 0,4- 0.5 of an ohm.  You would have trouble convincing me that any large trips (cooker circuit, shower)  will  be sure to meet the prompt disconnection requirement if there is a fault,  without help from an RCD unless the indoor wiring is very short. (we like to see Zs low enough to be sure of a fault current of rather more than 5 times the breaker rating to  ensure a rapid break, in this case most of the Zs allowance seems to be used up outside.)

    Still it means you will be able to easily see the kettle being plugged in, as the light will dim visibly. ?

    An additional test, if you have the patience, would be to look at the L-E drop and then the N-E rise when a known load is added and removed (that kettle would do).  That will tell you if N and L are both undersized but the same resistance, suggesting a thin cable that has always been wrong, or if there is a higher resistance in only one or the other - more likely to be a fault. (And if you are not sure if it is really TNS or PME in the street presenting as TNS, which is quite common after repairs or additions to the mains, then the CPC earth to true terra-firma earth voltge will bounce with load if the NE link is on your side of the neutral voltage drop.

    Are other buildings on the same substation affected or is it just this one ?


    Older buildings may have originally  had a 'lighting only' supply that has been modernised with only a new cut out fuse, rather than a new cable.
  • It is possible that the supply is restricted to only 40 amps. Or is MEANT to be 40 amps and that someone has improperly fitted a larger fuse. Not very likely these days, but 40 amp supplies do still exist. I found a 25 amp service recently.

    The voltage might be just within tolerance at 40 amps.
  • Or could just be a loose connection somewhere. I was asked to look at one house where 'the lights dimmed when the kettle was switched on' - loop readings were all over the shop but the give away was the audible 'crackling' sound coming from the DNO's cutout when the kettle was on. A call to the DNO with "my electrician says..." apparently brought a swift resolution.

      - Andy.
  • Suresly AJJ when doing a periodic/EICR you use your six senses? I thought everyone put the kettle on and listened to the cutout! If not they will now.

    Nice observvation on your part
  • Ebee and AJJ this reminds me that we had a loose cut out fuse here at Kelly towers it ran a bit warm and crackled with the kettle or immersion heater on I foned  the DNO who didn't really want to attend so I said can I cut your seals and do it myself oh no don't do that they said  one of there blokes was at the door in 15 minutes amazing isn't it? If you concentrate there minds service is quite good.
  • Thank you all for you input, it’s helped a lot. I’ve also had the DNO confirm 100 amp’s approved, although how they can expect voltage to stay in limits with a supply cable impedance near the TNS 0.8 ohm limit, I don’t know. For the installation I’m looking at (for a friend, so just out of interest in my retirement), the current demand, by domestic standards is large, and includes heat-pumps for both house and pool, and these in particular are being adversely affected by the low voltage. There are implications too re the COP of heat pumps and lots more with at these low voltages, so I have just submitted a comprehensive technical report to the DNO and asked for an urgent response. I also have some questions about the accuracy of Ze and pfc measurements on a loaded network, using standard loop testers, but I’m currently considering that separately.

  • With a confirmed supply capacity of 100 amps, it now seems clear that the DNO are failing to supply the required voltage and must correct this.

    Heat pumps, being a type of refrigeration compressor are particularly vulnerable to low supply voltage, as are fridges, freezers, air conditioners, and de-humidifiers and similar equipment.


    Many other loads will be fine. Resistance heating will give a much reduced output, but not suffer any harm. Modern LED lamps should be fine down to under 200 volts, with some types operating down to about 100 volts.
  • Former Community Member
    0 Former Community Member
    the current demand, by domestic standards is large, and includes heat-pumps for both house and pool, and these in particular are being adversely affected by the low voltage.


    Has to requirement to notify the DNO of the wish to connect a "disturbing load" over 2? kW and the consequences of "flicker" been rescinded?


    Regards


    BOD